The present invention relates generally to surface emitting lasers and more specifically to formation of an n-drive p-common laser fabricated on an n-type substrate.
Originally semiconductor lasers were diode structures where the light emitted from the edge of the laser structure was parallel to the surface of the semiconductor wafer. Unfortunately, this edge emitting laser structure does not lend itself to the cost-effective fabrication of two-dimensional arrays of laser diodes. A second class of laser diodes, well suited for fabrication of laser arrays, is fabricated such that the laser structure is perpendicular to the surface of the semiconductor wafer so that the emitted light is perpendicular to the surface. These laser diodes are commonly known as surface emitting lasers (SELs).
Both classes of lasers are formed on a starting substrate which may be either semi-insulating, p-type or n-type. Referring to FIG. 1A shows a cross-sectional view of a conventional n-drive SEL 100 formed on a semi-insulating substrate 102. The surface emitting laser 100 may be viewed as a n-i-p diode comprised of an n-type mirror region 104, an active region 106, and a p-type mirror region 108. Electrical connection is made via electrode 110 formed on the top surface of the n-type mirror region 104 and electrode 112 formed on the p-type mirror region 108.
To make electrical contact to the p-type region 108, an etch is made through both the n-type mirror region 104 and the active region 106 to the p-type region 108. This is problematic since the p-type contact etch exposes epitaxial layers 104, 106, 108 which have a tendency to oxidize. Further, the p-type contact etch creates a non-planar structure creating device reliability problems and increasing manufacturing complexity. Further, defects added to semi-insulating substrates to make the substrate isolating reduce the reliability of the semiconductor laser device.
Referring to Figure lB, shows a cross-sectional view of a conventional n-drive surface emitting laser 120 formed on a p-type substrate 122. The SEL is comprised of a n-type mirror region 124, an active region 126, and a p-type mirror region 128. Electrical interconnections are made via electrode 130 formed on the surface of the n-type mirror region 124 and electrode 132 formed on the surface of the p-type substrate 122. The preferred method of formation for the n, i, and p-type regions is by molecular beam epitaxy. The only commonly available p-type substrate is zinc doped. However, at typical MBE growth temperatures, zinc outdiffuses causing unacceptable background concentration in the mirror regions 124, 128 and the active region 126. Further, zinc outdiffusion contaminates the molecular beam epitaxy chamber resulting in an additional cleaning step after each zinc contamination.
FIG. 1C shows a cross-sectional view of a surface emitting laser 140 formed on an n-type substrate 142. The SEL is comprised of a n-type mirror region 144, an active region 146, and a p-type mirror region 148. The SEL 140 shown in FIG. 1C is a p-drive SEL. Unlike the n-drive current driven SELs shown in FIGS. 1A and 1B, the p-drive SEL is typically voltage driven. Although current drivers for p-drive SELs exist, they are problematic. The available silicon pnp drivers typically have insufficient speed for current data rates of optical communication systems and GaAs pnp drivers are expensive.
However, problems are also associated with voltage driven p-drive SELs. Voltage driven p-drive SELs in SEL arrays require precise control so that the Vf of each individual laser in the array is uniform. Nonuniformities in V.sub.f require individually pre-biasing each individual laser in the laser array; thereby increasing the cost of the laser drivers. Of course n-drive SELs may be created from the structure shown in FIG. 1C by sawing between individual lasers and flipping the p-drive SELs. However, this eliminates the possibility of SEL arrays.
A method of forming an array of n-drive semiconductor laser on a n-type substrate is needed.